1. Field of the Invention
The present invention relates to improved pulverizing and grinding hammers used in hammer mills to mechanically impact material fed into the mill, thereby reducing the size of the material particles. The improved hammer has a substantially trapezoidal insert made of a hard metal such as STELLITE 12 that is brazed onto a hammer head. The insert forms a contact face having an approximately ten degree layback angle with the vertical. The improved hammer has a longer useful life and can be used more efficiently than conventional hammers. The improved hammer also provides a clear boundary between the insert material and base metal of the hammer, which acts as a wear indicator to indicate when the layback angle wears to approximately 40 to 50 degrees, the critical angle at which mill pulverizing and grinding efficiency decreases.
2. Description of the Related Art
Hammer mills pulverize and grind materials as diverse as coal, minerals, sugar, pharmaceuticals, and food. A hammer mill contains a number of hammers that are each attached to a rotor at one of the pivot points spaced along the circumference of the rotor. When the rotor rotates, the hammers extend radially from the rotor due to centrifugal force, striking and pulverizing material fed into the mill. When the rotor is rotating and the hammers are radially extended from the rotor, there is little clearance between the top of the hammers and the liner on the upper mill chamber. Larger particles fed into the mill are impacted by the hammers and may also enter that clearance space and become ground between the top of the hammer and the mill liner. The particles exit the mill when they are ground to a size that will pass through a screen in the bottom of the mill. General descriptions of hammer mills are provided in U.S. Pat. No. 2,488,799 to Bonnafoux, and U.S. Pat. No. 2,316,124 to Sheldon.
The hammers in a hammer mill may be U-shaped or stirrup-type structures formed by a head and two legs extending down from the head. A hard face is formed on the front of the head where the head contacts the material to be pulverized and ground. The hard face of the head may be formed from a cobalt based alloy such as STELLITE 12 or STELLITE 6, or may be formed from an iron based alloy. STELLITE 12 and STELLITE 6 are registered trademarks of the Stoody Deloro Stellite Company. The rest of the head and the legs of the hammer are usually made of other materials such as carbon steel and stainless steel to reduce material costs and allow easy formation and better impact resistance. A general description of stirrup-type hammers is provided in U.S. Pat. No. 2,827,242 to Sheldon.
Two known methods of preparing the hard face of a hammer are to apply molten material to the head by conventional welding or plasma transfer arc welding, and then grinding the welded face material to a flat surface that forms an angle with the vertical. This angle between the vertical and the face is known as the layback angle. Test results show that a layback angle prevents a problem known as windage, in which less feed material is ground and the mill operates at a higher, less efficient temperature due to air turbulence. A hammer mill operates more efficiently, i.e. at a higher capacity and a lower temperature, when the layback angle is about 7.5.degree.-15.degree. rather than 0.degree..
In ordinary use, the hard face of the hammer wears more quickly near its top such that the layback angle increases with use. This wear pattern occurs because most of the material to be ground is concentrated in a shallow layer around the inside of the chamber, and because some of the larger particles of the fed material are ground by the top of the hammer against the liner on the upper mill chamber. It is therefore desirable to have a hammer with more face material near the top of the head for longer hammer life. In order to place more face material near the top, hammer manufacturers typically weld additional material at the top which results in an inverted triangular cross sectional area for the hard face material of conventional hammers.
Without increasing the size of the head it is not possible to weld excessive amounts of face material to the head, since to do so results in the weld material overflowing around the head and burning through the back of the head opposite the face. Conventionally sized hammers with welded faces are therefore limited in the amount of layback angle that can obtained with wear, and have a correspondingly limited useful life. Increasing the size of the head to allow for the application of more weld material is not desirable since that would increase the weight of the hammer and require more power to run the mill. Even if the size of the head is increased, the problem of material overflowing around the head remains.
When the hammer face wears off from use, the softer base metal of the head becomes exposed. If the mill is operated with the base metal of the hammer exposed, pulverizing and grinding efficiency decreases dramatically. By operating in this manner, the hammer mill is also more likely to break down due to vibration, resulting in expensive repairs and downtime. Users therefore need to remove and replace hammers before the base metal of the head is exposed. Users also want to incur minimal maintenance costs and interference with operation, and therefore want to use a hammer for its full life, removing it only when substantially all of the face material is worn away from the head.
To ensure that the base metal is not exposed during mill operation, resulting in decreased pulverizing and grinding efficiency and possible mill breakdown, users must determine when to remove and replace a hammer by visually estimating when the face material has worn away. It is difficult to visually estimate when a welded face has worn away, since welding results in significant penetration of the weld material into the base metal of the head such that there is no uniform visible boundary between the face and the base metal. This is true with both conventional and plasma transfer arc welding. Users therefore cannot accurately determine when the hammer is nearing the end of its useful life. Welding is also subject to operator control and error, which often results in porosity and shrinkage in the welded area and corresponding weakness in the hammer.
Accordingly, there is a need to provide an efficient, long-lived, hard-faced hammer for a hammer mill with a wear indicator which can accurately indicate when the hammer is nearing the end of its useful life.
It is therefore an object of this invention to provide an improved, efficient, long-lived, hard-faced hammer for a hammer mill.
It is also an object of this invention to provide an improved hammer for use in a hammer mill which is of conventional size.
A further object of the invention is to provide a hammer for use in a hammer mill with a wear indicator which can accurately indicate when the hammer is nearing the end of its useful life.
Another object of this invention is to provide a method of making an efficient, long-lived, hard-faced hammer for a hammer mill.